Electrical measuring apparatus



Nov. 4, 1952 J R, CANNON 2,616,948

ELECTRICAL MEASURING APPARATUS Filed Nov. 22, 194e 's sheets-sheet zINVENTOR. v

JAMES ROWAN CANNON.

A7' TORNE Y.

Nov. 4, 1952 J. R. CANNON 2,616,948

ELECTRICAL MEASURING APPARATUS Filed Nov. 22, 194e s sheets-sheet s VH@A50 TIE.. F15-1 E JAMES RowAN cANlgoN rwentor (Ittorneljf Patented Nov.4, 1952 ELECTRICAL MEASURING APPARATUS James Rowan Cannon, Liverpool,England, assignor to Automatic Telephone & Electric Company Limited,Liverpool, England, a British company Application November 22, 1948,Serial No. 61,456 In Great Britain December 5, 1947 (Cl. F75-1183) sclaims.

The present invention relates to improvements equipment which may beused in carrier telephone systems. In that arrangement alternatingcurrent at a predetermined frequency is applied by way of two circuits,one of known characteristics and the other to be tested, to a socalledsensitive detector embodying an electronic switching device and anindicating device such as a cathode-ray tube. The switching device,automatically and continuously switching from one circuit to the other,extends the output current signals from the two circuits alternately tothe cathode ray tube.` 'The sensitive detector is so arranged thattraces representative of the signals and consequently of thetransmission characteristics of the two circuits appear on the screen ofthe tube in juxtaposition so as to permit direct comparison of thecharacteristics.

The present invention is concerned with the combination of theabove-described sensitive de-v tector with suitable circuit arrangementsto provide a composite measuring apparatus for field and factory testingof equipment and components for use in carrier telephone equipment orthe like. Broadly speaking, therefore, the object of the invention is toprovide means for simply and rapidly determining, at any frequencywithin a predetermined range, the modulus of an impedance and the returnloss of one impedance against another.

According to one feature of the invention, a measuring set fordetermining the modulus of an impedance comprises a circuit having twobranches fed from a common source of alternating current each branchincluding an impedance,

said impedances having a known ratio one to the other while one branchincludes a calibrated variable standard impedance and the other includesthe impedance to be measured, the voltage developed across said rstmentioned impedances being applied to a sensitive detector of thecathode ray oscilloscope type in such a manner as to` enable thestandard and unknown impedances to be compared from the traces appearingin juxtaposition upon the screen of the cathode ray tube. According toanother feature of the invention,

a measuring set is providedfor the comparison of two impedances in termsof their return loss in suitable transmission units. Return loss is thepower reflected or returned toward the sending end of a system by reasonof an impedance mismatch between the sending and receiving equipment,and may be defined as the ratio of the power available for delivery to amatched impedance to the reiiected power. Such a measuring set comprisesa circuit having two similar branches, each including one of theimpedances to be compared supplied from a common source of alternatingcurrent and including transformers so arranged that signals indicativeof the difference current in the branches are applied directly to onepair of input yterminals of a sensitive detector whilst signalsindicative of the'sum of the currents in said branches are applied toanother pair of input terminals of the sensitive detector by way of avariable attenuator calibrated in suitable transmission units, saidattenuator indicating the return loss directly when adjusted until thetraces representing said signals and appearing in juxtaposition on thescreen of the sensitive detector are of equal amplitude.

The invention will be better understood from the following descriptionof various embodiments taken in conjunction with' the accompanyingdrawings.

In the drawings, Fig. l is a circuit of the impedance measuring setwhile Fig. 2 shows one method of suitably screening the impedancemeasuring set. Fig. `3 is the circuit of the return loss measuring setfor unbalanced impedances and Fig. 4 is a measuring set of similar typefor balanced impedances. Fig. 5 is a block diagram of asensitivedetector as described in detail in said British specication No.569,279. Fig. 6 indicates the appearance of the display screen of such asensitive detector when two unequal signals are being compared and Fig.7 its appearance when the signals compared have been brought toequality.

The circuit of the impedance measuring set shown in Fig. l may be `usedfor the measurement of the modulus of Vimpedances' when speed and easeof manipulation, rather than extreme accuracy, are the primerequirements as in the case of measurement of transformer and chokecharacteristics when the general shape of the impedance/frequency curveis usually of greater importance than very accurate impedancemeasurement at any particular frequency. Referring to Fig. 1 thevariable frequency oscillator is connected to a circuitarrangementcomprising four arms, R which is the standard decaderesistance tive spoolsfhaviiig ls'irr-lilar;phase; angles and being,t

of equal 'D. C." resistance so that'the ratio of their impedancesremains substantially constant and equal to unity. If need be the ratioof the impedances may be other than unity, providing that the ratio isknown and due allowance is made therefor. Resistors Z1 and Z2, areselected by means of a switch-adjustalole` potentiometer tapped forexample to give values of 0.1, 1.0,v

and 100 ohms, which range of adjustments will sui'lice for themeasurements'of' impedances. of l the order of 10 to 100,000 ohms. The'particular values chosen are dependent upon the unknown impedance and toobtain an accuracy of measurement within A2 per` cent,-` Z 1 should notexceed The sensitive detector SID which may be substantially similar tothat described in the prior specication No. 569,279 has one `pair' ofinput terminals IA connected across the resistor Z1 andthe other pair IBconnected across Z2.

' Such'a sensitive detector is illustrated in block form in Fig. 5. Thesignal fed to input IA is amplified by thev individual two-stageamplifier AI comprising valves VA 'and VB and after receiving furtheramplification' by thel common amplifier- AS comprising yvalves VG` andVH is applied to the vertical deflector' plates of the tube CRT. InputIB fis likewise ampliedV by ampliiiers A2 and A3 and is delivered to thesame pair of deiiector plates.

The outputs from amplifiers Al and A2 are, however, not connected4simultaneously to amplier A3, but alternately, this being achieved bymeans of the? multivibrator MV. comprising thevalves VC and' .VD whichkinteract in known manner sov that they are alternately conducting andnon-conducting andi which alternately bias the output valves VB. and .VEof each of the individual amplifiers beyond cutog. thus gating them in awell-known manner.

The. multivibrator MV alsoserves to control: the triggering of thetime-base valves VJ. sothat the-cathode ray tube sweep` frequency ispositively lockedy into the switching.` frequency. During thev firsthalf of any onesweep of the tube the vertical deiection is obtained' viaone individual amplifier and` during the secondil haltvia the otherindividual amplifier. Since the signal frequency willv usually be verymuch higher than the switching frequency which will conveniently be'oftheorder of SG1-cycles per second, the appearance ofthe picture on thetubescreen will thus be two illuminatedV rectangles of equal breadth as.shown inMFig,` 6, the height of the rectangle being directlyproportional to the magnitude of the applied voltages to the inputs IAand IB. With the two voltages made equal by suitable adjustment of avariable attenuator, as will later be described, the. appearance of thescreen is as shown inFig. 7.

The method of determining. the modulus of the unknown impedance at apredetermined frequency will now be described. Suitably low values of Z1and Zz are inserted and the appropriate frequency is applied by theoscillator. Now the same voltage V is applied across both branches ABand CD of the circuit so that if the currents in these branches arevrepresented 4 by i1 and i2 respectively and the unknown irnpedance as avector quantity I( then and that IR+ZII=IX+Z2| 1mi/ Hlm R-l-.Z1 +Z2However Z1 and Z2 were made negligibly small compared with R.. andXtherefore R=lX l. It will be apparent that the arrangement isconsiderably more simple and speedy in operation than the conventionaltype of impedance measuring bridge in that, once having chosen suitablevalues for Z1 and Z2 only one Iadjustment is necessary. This isparticularlyadvantageous inthe preparation of an impedance/frequencycurve. of a component. It wil1 further be apparent that had the ratioZ1:Z2 been other thanvunity, then the ratio R:[ would have been the samewhen the tracesA on the screen werey made equal in amplitude. Y Y

Fig. 2 shows one arrangement of the impedance measuring set suitably.screened tominimize the elect of stray couplingsv when high frequenciesare employed and/or very high impedances are to be measured, The screensare shown by broken lines and are: all connected to earth, but it isimportant to. note that any screen on the component to be `measured: orthe frame thereof should be connected to terminal i2, otherwise theeffect of the capacities to earth oi such screen or frame willbeincluded in the measurement. In. order to further minimize theliability of pick up itwill be of advantage to use alowimpedanceoscillator.v

The eiectozf the capacitances introduced by the screen may be:investigated as follows; The total capacity (CC)v to; screen of thatportion of the circuit from the. high. potential terminal of theoscillator and terminals. H; an'd l2 or R and X respectively is shuntedacross the oscillator outputand is consequently. ineffective. Thecapacitances due to the other screening resolve themselves into CA` andCB which are in eiect shunted across.. arms Z1 and Z2 respectively.Since these arms are .of comparativelyy low impedance the shuntingleffect of the capacitances may be ignored for the majority of,measurements, particularly if-care is, taken in the physical lay-out.of` the. measuring set components to reduce CA and CB to the smallestpossible values. Further if necessary a small capacitor may beintroduced to. obtain an accurate,- balance between CA. and-CB.-

It may beY pointed-out` that for the measurement 0i.- highl impedancesdtmay be convenient to replacethe resistance box R by a standard lowreadingvariable capacitor.

ReferringA nowV toY Fig.. 31 whiclrisv av circuit for a` return` lossmeasuring set, and is in` eiect` a means; for comparing., anunbalancedimpedance expressed as avector quantityZr with. av standard impedancevZ3 `frequently but notnecessarily non-.reactiveand for expressing thedegree of out of balance of the two. impedances in decibels. Theapparatus will ilndl particular application in carrier telephony indetermining the characteristics ofv input and'lor: output impedancesA ofequipments' such as modulators, amplifiers andV attenuators forimpedance matching purposes.`

First itshould be noted that `the return loss at a] given `frequency ofan impedance expressed as a vector quantity Zrfagainst another impedanceZa has been defined by l Y `Now transformers TRA and TRB are of the typenormally employed as hybrid transformers in transmission circuits andare of substantially identical construction and have turns ratios suchthat the impedance introduced into the circuit branches by way ofwindings b and c is negligible compared with the values of Z3 and Z4.The low impedance-primary winding b and c of the transformer TRAcomprise a twin twisted pair of conductors and are consequentlysubstantially equal and each have equal turns ratios, of the order 1 andthese components are connected one in eachv of the substantially similarbranches of the` measuringset. The variable frequency oscillator OSC setat a vpredetermined .frequency is connected in the path servingbothbranches and produces a voltage V across points P and Q, andv thebranch currents expressed vectorially as is and irow through Zaand Z4respectively.

Transformer TRA winding a connects directly with a pair of inputterminals IA of the sensitive detector SD which is of the typepreviously re-v ferred to.V Transformer TRB winding f extends to theother pair of detector input terminalsIB by way oft-the variableattenuator VA.

Now

t. V V @FZ and @FZ since the impedance introduced into the branchkcircuits by TRA is negligible. It will be assumed that transformerwindings a and f terminate on impedances of substantially 600 ohms andthat the turns ratios of windings. b to a and d ,to f are each `1 in.Since windings b to c are in series opposition then, across winding a,the potential difference and, since windings d and e are parallelaiding, across winding f the potential difference To determine thereturn loss the attenuator.

VA, which is calibrated in decibels, is now adjusteduntil the two inputvoltages to the sensitive detector are equal as indicated by theVequality of amplitude of the traces appearing in juxtaposition on thescreen of the detector.

"Assuming that an attenuation A decibels isdnv A lOglo 2o log10 whichequals the return loss at'the predetermined frequency of Z4 against Z3in decibels. It follows that the ,attenuator indicates the returnV loss*directly. t

It will be apparent that, by applying the abovev methods, a returnvloss/frequency curve may 4be derived in a few minutes by simply settingthe oscillator successively at each of a plurality of frequenciescovering the working range of the component to bev tested and byadjusting the attenuator untilthe traces appearing in juxtaposition onthe screen of the cathode ray tube are equalfin'amplitude.

Referring now to Fig. 4 which shows the return loss measuring setadapted for determining the return loss of a pair ofimpedances Z5 and Zseach balanced with respect to earth.

Impedance Z5 represents the standard balanced impedance while'Ze may bea balanced transmission line or the input or output imped-` ance of thecomponent whose return loss against the standard impedance is to befound.

The substantially lsimilar transformers TRD and TRE correspondrespectively in vfunction vtov transformers TRA and TRB referred to incon` nection with Fig. 3 while transformer TRC serves to couple theoutput'of the variable frequency oscillator OSC equally to the upper and'lower portions of the centre limb of the circuit including the Jprimarywindings e2, d2, e1 and d1 of transformerTRE. The latter primarywindings have equal characteristics and are connectedin the same sense.The primary windings b1, c1, b2 and c2 of transformer TRD also haveequal characteristics but are connected in such a mannerv that bz and czoppose and' b1 and c1 also oppose, while b2 and ln assist and c2 and c1also assist.

It will be apparent that the upper and lower portions "of the circuitsembracing all'the pri-` mary windings. of transformers TRD and TRE areidentical and that the" voltage appearing across the secondary `winjdinga1 of TRD is indioative of the totalcurrent differences in Z5l and Z5while the voltage appearing across the secondary winding fr of 'I'RE isindicative of the sum of the currents flowing in Z5 and Z5. The methodof determining the return loss is similar to that "described inconjunction with Fig. 3 and relating to unbalanced impedances. Assume asin the case of Figure 3 that the transformer windings a1 and f1terminate on impedances of substantially 600 ohms and that the turnsratio of windings b1 and b2 to a1 kand d1 and d2 to fris lzR. Then Theattenuator VA is .adjusted as described for.

Figure whereupon the attermation:.ivxitrfi-1VV duced is A decibels, itcan be shownima exactly the same way as indicated for Figure 3 that thecathode ray` oscilloscope type` providing;V

means for direct comparisonv of the amplitude of two signals applied totwo sets of inputK terminals, the transformer coilv excited by thedifference currents being connected to oneof' said sets and thoseexcited bythe sum currents to the other, and'. a variable attenuatorcalibratedin suitable transmission units interposed between thesum-excited coils. and said other Sel?v Of terminala. Said. attenuatorindicating the return loss directly when adjusted until theA tracesrepresenting said signalsl and"Y appearing injuxtaposition on the screenof" the sensitive detector are of' equal. magnitude. 2'.V Ameasuringapparatus, as claimed incl'aim L in which the transformer meansare hybridtransformers of the type used in transmission circuits. y n v3. A measuring set as claimed` in claim l' in` which a primary windingvof one ,transformer means is arranged in each` ofsaid branches', andAthe common source of. alternating current` in series with the. parallelconnected primary wind-I ings ofanother. transformer. meansis, connectedbetween the junction of the two impedances and.` the junction of thefirst-mentioned `Illiirnary windings whereby the first-mentionedprimary' windings'are in series opposition and the secondimentionedprimary windings are.. in the; sameA sense, 4. A measuringset; as.claimed in claim 1 and adapted for the. comparison of. two. balanced'impedancesin terms ofthei'r return loss.in decibels wherein saidtransformer means comprises a..p air of hybrid transformers. ofthctypeusedin transmission circuits wherein one transformer has'two" pairs ofprimary windingsand thesecondtransformer has two pairs. of'parallel-connected primary windingsY in which one primary, windingoffeach of. two. pairs. of primary windings. oigon'e transformer isarrangedon each. side ofjtheimpedance in each ofsaild branches and.' thetwo pairs of parallel-connected primary, windings'of the secondtransformer in serieswithandone. on' each side ofY the connnonsource ofalternating. current are. connected. between thev J'imctions of' the.pairs. of. first-mentioned primary windings, whereby the pairs. of.lErst-mentioned primary windings. arein seriesoppositi'on2,gnctthepairsof second-mentioned. primary. windingsgare in. the. same.sense. y y y 5. Measuring apparatus` for the.. comparison.o of twoimpedances in terms-ofy their. return loss. in suitable transmissionunits comprising a circuit having two similar. branchcsfreaoh includingone of' the impedances to be compared, a source of alternating currentincluded in said circuit,

8*, windings-included said circuit and its, secondary winding arrangedto respond to the difference in currentvowin said branches,y at leastone fur.ther transformen having itsy primary windingsVi-ncluded; in,said circuit and its secondary winding arranged to respond to. the sumof the currents flowing in said branches,l a sensitive detectorincluding a cathode ray tube and having two pairsrof input terminals,means for connecting the secondary winding of said one transformer toone-.pair of said input terminals, a.. A.variable attenuator calibratedin suitable transmissionunits andi means,- for. connecting the secondarywinding of said further trans.- former through saidattenuator, to theother pair of said input terminals. whereby said attenuator indicatesthel return loss. directly when adjusted untiL thetracesrepresenting.the. outputs from saidtwo transformers and appearing injuxtapositiononthe4 kscreenfof said tube. are of equal amplitude. l-

6.- Measuringv apparatus. for the. comparison of. two impedances` interms of. theirreturn loss. in. suitable transmission units. comprisinga circuit havingf twosimilar branches each including one ofy theimpedances tobe compared, and a third. branch connected in parallel withsaid two branches, a -source of alternating current included-inv saidthird branch..y a. first. transformer having two primary windingsconnected in paralleiA and inY the same-sensein' saidth-ird: branch, asecond.g transformer having. two primary windings. connected in seriesopposition. in said two branches. a sensitive detector including acathodefray.y tube; and having rst and second: pairs of: input.terminals',y means; for connecting the secondary winding off sai-d..second transformer: to.said first pair-cf.` saidfinput terminals,. avari.- able attenuator calibrated in suitable-transmission unitsandmeans forv connecting` the. secondary wind-ing of said rst transformerthrough said.'r attenua-tor to said-second pair of input. terminalswhereby said attenuator, indicatesI the return' loss-f. directly` whenadjusted until the tracesi representing the outputs from said rst andsecond transformers and. appearing; in juxtaposition on the screen ofsaid tube are of equal amplitude.

7. Measuring apparatus for the comparison of two impedances in terms:oftheir return loss in suitable transmission units comprising a circui-thaving twofsimilarl branches each including one-ofthe impedancesto becompared and a third? branch connected: parallel' with said. two`branches, a= first. transformer having two pairs ofprimary windingsinsaidfthird'lbranch,

each pairconnectedin parallel and the two pairs connected in.,y the`same. sense. at` source:of alter.- nating current, a second transformerhaving its` primary winding connected in circuit with said source andits secondary winding connected in said third branch between the twopairs of primary windings. of, said `rstA transformer,1 a, thirdtransformer having, two pairsof.. primaryv wind'- ings, each pairconnected'in seriesopposition in said two branches, a sensitive detectorincluding a cathode ray tubefand having' two pairs of input terminals,means for connecting the secondary/.winding of said;third.transformer toone pail: off. said input terminals; a variable: attenuatorf calibrated@suitablestransmission units. and means;` for.` connectingf the:secondary winding of saidf first transformerY through said..v attenuatoyto the other pair -ofinput terminalsmhereby said:

at least. one transformer-'having itsiprimaay71s.:attenuatorwindicateswtheV returnl loss.` directly vwhen adjusteduntil the traces representing the output from said first and thirdtransformers and appearing in juxtaposition on the screen of said tubeare of equal amplitude.

8. Measuring apparatus as claimed in claim 7 wherein said transformerscomprise hybrid transformers of the type used in transmission circuits.

JAMES ROWAN CANNON.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Number

